1. Field of the Invention
The present invention relates generally to transparent conductive materials and more particularly to barium copper sulfur fluoride transparent conductive thin films and bulk materials.
2. Description of the Prior Art
Films consisting of transparent conductors (TC's) with p-type charge carriers (p-TC's) are desirable for several applications including multijunction solar cells, transparent electronics, low operating voltage light-emitting diodes, high-efficiency solar cells, and electro-magnetic interference (EMI) shielding. Moreover, transparent conductive coatings that can be deposited on plastic substrates are desirable for many applications, for example, wearable computers, lightweight displays, higher-efficiency emissive displays, large-area displays, and improved solar cells.
Current p-TC films have several problems. Most materials that exhibit p-TC behavior are poor conductors with typical conductivities of <1 S/cm. The best current p-TC films have conductivities as high as 220 S/cm at room temperature, but these films require high processing temperatures (at least 400° C.). Therefore, they are incompatible with organic optoelectronic materials and exhibit interdiffusion when heated while in contact with other inorganic films. In addition, many existing methods of forming high-conductivity p-TC films require the use of single-crystal substrates such as MgO, making them impractical for most applications.
Barium copper sulfur fluoride (BCSF) is a crystalline material that, in the prior art, consists of alternately stacked fluorite and anti-fluorite layers that form a tetragonal crystal structure in space group P4/nmm. Bulk BCSF compound is intrinsically p-type, and when doped with potassium, it has been shown to possess bulk conductivity as high as 100 S/cm in selected crystals.
One method for producing BCSF thin films is described in the following reference, the entire contents of which are incorporated herein by reference: Yanagi, Park, Draeske, Keszler, and Tate, “P-type conductivity in transparent oxides and sulfide fluorides,” J. Solid State Chem., 175, 34-38 (2003) (hereinafter referred to as the Yanagi article). As described in the reference, the authors co-evaporated Cu metal and BaF2 onto SiO2 and MgO substrates. They then treated the films under flowing H2S gas. The resulting films had conductivities of only ˜0.1 S/cm at room temperature.
Bulk BCSF
There are currently two general methods for fabricating bulk BCSF. The first method entails heating precursors in a sealed quartz ampoule. As described in Zhu, Huang, Wu, Dong, Chen, and Zhao, “Synthesis and crystal structure of barium copper fluorochalcogenides: [BaCuFQ (Q=S, Se)],” Materials Research Bull., 29, 505-508 (1994), the entire contents of which are incorporated herein by reference, precursors CuS, BaS, Cu, and BaF2 were pressed into pellets and heated to 450° C. for 12 hours in a sealed quartz ampoule. The second method does not make use of a sealed ampoule. Some examples of the second method are described in the Yanagi article. In one example, the precursors Cu2S, BaS, and BaF2 were heated to 450° C. for 15 hours. In another example, BaCO3, Cu2S, and BaF2 are used as precursors. In these examples, the precursors were heated to 550° C. for an unspecified period of time under flowing H2S gas in a refractory boat rather than a sealed ampoule. The following article, the entire contents of which are incorporated herein by reference, describes another example in which precursors BaCu2S2 and BaF2 were heated to 650° C. for 15 hours: Park, Keszler, Yanagi, and Tate, “Gap modulation in MCu[Q1-xQ′x]F (M=Ba, Sr; Q, Q′=S, Se, Te) and related materials,” Thin Solid Films, 445, 288-293 (2003).
None of these methods produces good quality BCSF. The quartz ampoule method results in residual quantities of the precursors within the bulk material. This effect may be mitigated by pressing the precursors into pellets prior to baking. Although this technique helps, it does not solve the problem. In addition, this technique adds an extra processing step and may be impractical for large quantities of material. Furthermore, placing the precursors directly in contact with the quartz ampoule can result in chemical reactions between the precursors and the ampoule and lead to contamination with oxides. The second method requires treatment under flowing H2S gas—a step that is potentially hazardous and may be impractical for large quantities of material.
P-TC Thin Films on Plastic Substrates
Both p-TC's and TC's with n-type charge carriers (n-TC's) are desired in order to form a variety of transparent electronic and opto-electronic devices. While several n-TC's that can be deposited on plastic substrates are well-established, such as indium tin oxide, zinc oxide, and amorphous In—Ga—Zn—O, there are no available p-TC's with comparable conductivities.
Currently, there are only a few p-TC's that are compatible with plastic substrates. One existing approach is to use organic thin films. While this approach has resulted in both n-TC's and p-TC's, their conductivities are generally lower than desirable, and these materials are typically subject to environmental degradation. Another possible approach is to use an inorganic material. Most existing inorganic p-TC's require deposition at high temperatures (>300° C.). Some require single-crystal substrates that act as a template for crystalline film growth. Either of these requirements makes the use of plastic substrates impossible with the exception of the amorphous semiconductor, a-ZnO.Rh2O3, as reported in Harushima, Mizoguchi, Shimizu, Ueda, Ohta, Hirano, and Hosono, “A p-type amorphous oxide semiconductor and room temperature fabrication of amorphous oxide p-n heterojunction diodes,” Advanced Materials, 15, 1409-1413, the entire contents of which are incorporated herein by reference. However, these films possess electrical conductivity of only 2 S/cm, approximately three orders of magnitude lower than that of indium tin oxide. Higher conductivity p-TC's are needed for efficient, low operating voltage devices.